It is commonly known that directional solidification can enhance the structural properties of a cast part. Accordingly, it is desirable to cool a mold from one or several distinct locations. It is known in the prior art to cast materials, such as metals, within a pressure vessel. For instance, see U.S. Pat. No. 5,111,870. This method consists essentially of disposing a mold and a source of molten metal, within a mold. The vessel is then pressurized in a manner which forces the molten metal into the mold. It is also known in the past to effect directional solidification within the pressure vessel by thermally contacting the mold with a cooled member such as a chill plate. This is also shown in U.S. Pat. No. 5,111,870. However, there is no apparatus or method disclosed in the prior art which introduces a fluid, such as gas, into a pressure vessel such that a thermal gradient is formed within the gas and across the mold which causes directional solidification of the material within the mold. Casting methods such as hot isostatic pressing (HIP) do not induce a thermal gradient since the gas pressure and temperature are raised at the same time.
The formation of a thermal gradient is similar to the process of hot air rising in a room. Gas that initially enters the pressure vessel with a heated mold and after heating is terminated, is heated more than subsequently introduced gas and thus rises to the top of the pressure vessel while the cooler gas moves to the bottom of the pressure vessel. The cooler the gas is, the lower it is disposed in the vessel. In a room at 1 atmosphere pressure, a temperature gradient exists. Raising the gas pressure increases the temperature gradient. The higher the gas pressure, the higher is the gradient. One would expect the temperature would be more uniform with higher pressures due to increased conductivity, however, this is the opposite of what occurs.